Iodide and sulfite synergistically accelerate the photo-reduction and recovery of As(V) and As(III) in sulfite/iodide/UV process: Efficiency and mechanism

Abstract

Photo-reduction of arsenic (As) by hydrated electron (e_aq^-) and recovery of elemental arsenic (As(0)) is a promising pathway to treat As-bearing wastewater. However, previously reported sulfite/UV system needs large amounts of sulfite as the source of e_aq^-. This work suggests a sulfite/iodide/UV approach that is more efficient and consumes much less chemical reagents to remove As(III) and As(V) and recover valuable As(0) from wastewater, hence preventing the production of large amounts of As-containing hazardous wastes. Our results showed that more than 99.9% of As in the aqueous phase was reduced to highly pure solid As(0) (>99.5 wt%) by sulfite/iodide/UV process under alkaline conditions. Sulfite and iodide worked synergistically to enhance reductive removal of As. Compared with sulfite/UV, the addition of iodide had a substantially greater effect on As(III) (over 200 times) and As(V) (approximately 30 times) removals because of its higher absorptivity and quantum yield of e_aq^-. Furthermore, more than 90% of the sulfite consumption was decreased by adding a small amount of iodide while maintaining similar reduction efficiency. Hydrated electron (e_aq^-) was mainly responsible for As(III) and As(V) reductions and removals under alkaline conditions, while both SO₃^•- and reactive iodine species (e.g., I^•, I₂, I₂^•-, and I₃^-) may oxidize As(0) to As(III) or As(V). Acidic circumstances caused sulfite protonation and the scavenging of e_aq^- by competing processes. Dissolved oxygen (O₂) and CO₃^2- prevented As reduction by light blocking or e_aq^- scavenging actions, but Cl^-, Ca²⁺, and Mg²⁺ showed negligible impacts. This study presented an efficient method for removing and recovering As from wastewater.

Keywords: Efficiency; Elemental arsenic; Hydrated electron; Photo-reduction process; Sulfite/iodide/UV synergistically process.